Ultra high frequency system



Dec; 10, 1946. c. DE w ULTRA HIGH FREQUENCY SYSTEM Filed Oct. 31, 1942 -Fig.3.

Inventor: Kenneth iii Walt,

y H is Attorney Patented Dec. 10, 1946 urinal) sr ULTRA HIGH FREQUENCY SYSTEM Kenneth C. De Walt, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application October 31, 1942, Serial No. 464,036

10 Claims.

My invention relates to ultra high frequency systems, and more particularly to dielectric wave guides of the hollow-pipe type and associated apparatus for propagating or attenuating the electromagnetic Waves within the guides.

It i now rather generally understood that electromagnetic waves may be propagated through the interior of a metallic member or pipe of conductive material which contains a. dielectric medium, if the frequency of the exciting electromagnetic Waves is greater than a critical or minimum frequency, the value of which is established principally by the transverse dimensions of the guide structure.

In accordance with the teachings-of my invention described hereinafter, I provide new and improved structure of a device used in conjunction with a dielectric wave guide and which may be employed as a means for selectively obtaining propagation or attenuation of the waves within the guide. More particularly, in accordance with the teachings of my invention I provide a new and improved enclosing structure for a resonant aperture or slot to be positioned in or associated with a dielectric wave guide, and which is readily adaptable for rapid manufacture and assembly.

It is an object of my invention to provide a new and improved ultra high frequency dielectric wave guide system.

It is another object of my invention to provide a new and improved sealed enclosure for a resonant aperture or slot to be positioned in, or associated with, a dielectric wave guide of the hollowpipe type.

It is a further object of my invention to provide a new and improved sealing and enclosing structure for a resonant aperture or slot to be positioned within a dielectric wave guide of the hollow-pipe type, and which is inexpensive to manufacture and readily adaptable to rapid assembly by virtue of the relatively small number of parts involved.

Briefly stated, in the illustrated embodiment of my invention I provide in an ultra high frequency electromagnetic, wave guide of the hollowpipe type, a tuned or resonant aperture structure which is positioned in or associated with the guide and which comprises a metallic wall lying in a plane substantially transverse to the direction of wave propagation through the guide, and which includes therein an aperture or slot tuned substantially to an electromagnetic wave of predetermined frequency equal to or greater than the critical minimum or cut-ofl frequency of the guide. More particularly, the improved structure which I provide includes a metallic member, such as a cylinder, which extends through a circular opening in the transverse wall and also surrounds the tuned aperture or slot. The metallic cylinder extends an appreciable distance on each side of the wall to facilitate the sealin of dielectric windows, such as vitreous or glass Windows, t the cylinder, thereby establishing a hermetically sealed region about the aperture giving it thereby a high degree of sensitivity to the magnitude or intensity of the electromagnetic waves within the guide.

For a better understanding of my invention, reference may be had to the following description taken in connection with the accompanying drawing, and its scope will :be pointed out in the ap pended claims. Fig, 1 diagrammatically illustrates an embodiment of my invention as applied to a dielectric wave guide of the hollow-pipe type having a rectangular cross section, and Figs. 2 and 3 are a cross-sectional view taken along the lines 2-2 of Fig. 1 and an end view, respectively, of the enclosing structure for the resonant aperture.

Prior to the detailed discussion of my present invention as applied to wave guides wherein electromagnetic energy is transmitted dielectrically, it is believed that it may be helpful to review generally certain aspects of the phenomena associated with the propagation of electromagnetic waves through guides. As is well appreciated, wave guides of this nature may be employed for the transmission and propagation of electromagnetic waves of a variety of forms or character. These waves have been generally indicated heretofore as being of the E and H types. subscripts, as indicated, EM and Hn,m, have been assigned to these waves in order to indicate the nature of the waves, where n represents the order of the wave and m represents the modeofpropagation. The order. of the wave is determined by the manher in which the field intensity varies circumferentially around the axis of the guide, whereas the mode is determined by the manner of its variation withradial distance from the axis of the guide. The E type waves have both a longitudinal and a transverse component of electric field but only a transverse component of magnetic field. The H type waves have both a longitudinal and a transverse component of magnetic field but only a transverse component of electric field. Although my invention is applicable to systems, for the transmission of a great variety of waves of both the E and H types, in describing my invention 3 hereinafter reference will be made particularly to the H01 type of wave.

It is also appreciated that, in pipes for dielectrically guiding Waves, each configuration of the pipes establishes a critical minimum frequency below which energy is not propagated through the guide. Stated in other words, for frequencies below this critical frequency, the wave is rapidly attenuated so that energy cannot be transmitted through the guide. Above the critical frequency, the wave is propagated in the desired direction and the wave assumes a time-space distribution through the guide established principally by the wave length of the exciting impulses for the guide and the transverse dimensions of the guide,

Referring now to Fig. 1, I have there illustrated my invention as applied to a high frequency dielectric wave guide system for con-. trolling the propagation or attenuation of electromagnetic waves of predetermined frequency. Although my invention may be applied to dielectric wave guides of the hollow-pipe type having various cross-sectional configurations, for the purpose of facilitating the description of my invention I have chosen to consider a system which is used primarily for the transmission or attenuation of an H01 type wave within a wave guide having a rectangular cross section,

It is desirable that the metallic enclosure for the guide be of a metal of large electrical conductivity, such as copper or brass. The wave guide may comprise a metallic base plate I and a top plate 2 of transverse dimensions 1), and includes metallic side walls 3 and 4 having a height a. The improved enclosure for the resonant aperture slot to be described presently may be employed in a variety of ultra high frequency systems, andit is not necessary in the utilization of my invention to employ the particular wave guide construction illustrated. One way in which the structure may be employed is by using a wave guide in which one end thereof is terminated in a metallic plat 5 which may be welded or soldered to the base and top plates 1 and 2 and the side walls 3 and 4. The other end of the dielectric wave guide section as illustrated may be terminated in a flared horn structure 6 which may be employed either in the transmission or reception of electromagnetic waves. Alternatively, this end of the wave guide section may be terminated in a transverse metallic Wall and electrode means may be employed in place of the horn structure 6.

As a means for exciting or extracting high frequency electromagnetic energy from the wave guide, I may employ a concentric or coaxial transmission line 1 comprising an outer tubular and metallic conductor 8 which is conductively connected to the base plate I substantially surrounding a circular opening 9 in plate I. The other element of the concentric transmission line may comprise an inner conductor In which may be conductively connected to the top plate 3. Of course, it will be appreciated that I may employ other exciting or energy extracting means, such as probes of various configurations or the equivalent thereof.

I employ a metallic wall ll lying in a plane substantially transverse to the direction of wave propagation through the guide and which also may be. constructed of a highly conductive metal, such as copper or brass, and which is conductively connected to the members [-4, inclusive, of the guide. The wall [I is provided with a resonant apertur or slot l2 (see Fig. 3) which is tuned to a frequency equal to or greater than the critical minimum or cut-off frequency of the guide established. Although I may employ resonant apertures of various configurations, for the purpose of illustrating my invention I have chosen to represent the resonant aperture l2 as comprising a central elongated portion or slit l3 terminated at the ends thereof in enlarged openings, such as circles I l and I5. The aperture I2 is arranged to have its principal or longitudinal dimension substantially transverse to the electric component of the electromagnetic field due to the waves within the guide in order to efiect a concentration or acoentuation of the potential difference due to the waves. By virtue of such a configuration, or the equivalent thereof, the aperture or slot in its entirety is resonant to a predetermined frequency determined by the dimensions, principally its height and length. The slot, of course, has a distributed inductance and a distributed capacitance which resonate to the particular frequency for which it is designed. Consequently, the entire potential difference of the electromagnetic waves within the guide appears across the top and the bottom of the slot.

I provide a new and improved enclosing structure for the resonant aperture 1 2 and which may be more clearly appreciated by referring to the cross-sectional view shown in Fig. 2. A metallic member or cylinder I6 is provided which extends through a circular opening 11 in the transverse wall H. It will be noted that the cylinder l6 surrounds the aperture l2 and extends an appreciable distance on each side of the transverse wall I I. I also provid dielectric windows l8 and 19 which are sealed to the metallic cylinder l6 and may be preferably sealed to the inner surface of the cylinder. The dielectric windows l8 and i9 may be of the bulbous or substantially semi-spherical configuration, as illustrated, and may be constructed of a vitreous material such as glass and are sealed to the cylinder l6 by any of the now well known glass-to-metal seals such as a sealing structure comprising, as an alloy, a composition of iron, nickel and cobalt, which may comprise the cylinder l6.

By virtue of the configuration of the bulbous windows 18 and [3, the reflection of the windows or enclosure may be made substantially equal and opposite to any reflection incident to the presence of the transverse wall I I and associated resonant aperture, so that the total assembly is substantially refiectionless.

As stated above, the dielectric wave guide, by virtue of its configuration, is critically responsive to the frequency of electromagnetic waves at which it is excited, there being a minimum or cut-01f frequency below which the electromagnetic waves are rapidly attenuated so that the waves are not transmitted an appreciable distance through the guide. The wave lengthof the wave transmitted through a rectangular guide is a function of the wave length of the exciting wave in free space and the dimensions of the guide. For example, in a rectangular airfilled wave guide of base b and height a, the wave length )\g for an H01 type wave may be expressed where A is the wave length of the exciting wave in free space and where the electric component of the field is perpendicular to b.

5 The total impedance Z of a rectangular guide using air as the dielectric may be defined as follows:

In the embodiment of my invention illustrated, the change in guide dimensions or effective dimensions occasioned by the presence of the transverse wall H is made a small fraction of a wave length and any reflection due to the discontinuity so produced is inappreciable. Furthermore, it can be shown, analytically and experimentally, that the reflection may be cancelled by a similar discontinuity, opposite in sense, a very short distance along the guide from the first discontinuity. Considering the resonant aperture or slot and the effect thereof, the total impedance of the slot is the same as that of the guide since the reflection produced at the en trance of the slot is cancelled by the reflection from the exit. Furthermore, the assembly, including the wall H, the resonant aperture or slot I2 and the enclosing assembly, may be designed as a whole so that the resultant discontinuity or the reflection of incident waves occasioned by the assembly, may be made negligible or substantially zero.

No detailed analysis is presented here as to the manner in which the eifective length and height of the resonant aperture or slot i2 may be computed. Generally speaking, the relative values of the effective longitudinal dimension and the height of the aperture may be modified to estab lish the desired resonance of the slot for a wave of particular frequency. As the height of the aperture approaches zero, the effective longitudinal length thereof approaches the limit where A is the wave length of the electromagnetic wave in free space. For a more detailed analysis of the method of computing the dimensions of the resonant aperture or slot, reference may be had to Patent 2,467,068, granted September 3, 1946, to Milan D. Fiske and James M. Lafferty, and which is assigned to the assignee of the present application.

The medium within the enclosure comprising cylinder l6 and the bulbous windows l8 and i9, and which surrounds the resonant aperture I2, may be air at a. relatively low pressure or may be a suitable inert gas, such as neon, xenon, or argon. Alternatively, the medium within the enclosure may consist of a gas such as nitrogen in water vapor, or any other suitable gas which permits the establishment of an electric discharge, or an arc discharge, when the voltage difference between the horizontal dimensions of the slot attains a predetermined value and which also permits rapid extinction of the electric disconstant It charge, or are discharge, as soon as the voltage decreases sufliciently.

Some of the advantages incident to my invention are the simplicity of the structure which is readily adaptable to evacuation and assembly. Furthermore, by virtue of the metallic cylinder it, there is provided a simple construction to which may be sealed the dielectric or vitreous windows I8 and [9.

As to the method of assembly, it may be stated that the structure may be assembled in any one of several ways, one of which may comprise an initial sealing operation of one of the windows, such as window Hi, to the cylinder 16 which is then passed through opening il in the wall structure ll. Thereafter, the window 19 may be sealed in place in a chamber of desired pressure, or an atmosphere or medium of desired pressure. Alternatively, the window it may be provided with a tubulation (not shown) and after scaling to the cylinder 86 and after exhaustion and filling with the desired medium, the tubulation may be sealed. This tubulation may be such that after the sealing-off operation no substantial discontinuity or bulge is present on the surface of the window l9.

The operation of the embodiment of my invention shown in Fig. 1 will be explained by considering the system when it is operated as a means for selectively controlling the propagation or attenuation of received ultra high frequency ignals. In such an instance, the flared horn 6 of the wave guide may be employed as a means for receiving the signal. So long as the intensity of the received signal remains below a predetermined value, the transverse wall I l and the aperture l2 and enclosing structure oifer substantially no impedance to the propagation of the electromagnetic waves through the guide, and the electrode means 1 may be employed as a means for extracting a signal from the guide which is responsive to the electromagnetic waves therein. By virtue of the design of the resonant aperture I2, if the intensity of the electromagnetic waves increases to a value above the value established by the design of the aperture and the surrounding atmosphere, the voltage which appears across the horizontal edges of the aperture will exceed the breakdown voltage of the medium constituting the atmosphere to establish a discharge across the horizontal edges or dimensions thereof, thereby changing the aperture from a resonant device to a dissonant device. As a result, the transverse wall II and the associated aperture 12 constitute a substantial discontinuity within the guide relative to the propagation of the electromagnetic waves, and the waves will be rapidly attenuated so that the waves are not transmitted an appreciable distance towards the electrode means 1. Upon interruption of the received signal, the consequent removal of the voltage across the aperture will extinguish the electric discharge, thereby restoring the aperture to its; resonant condition.

Another way in which the embodiment of my invention illustrated in Fig. 1 may be employed is in a signalling or locating system wherein other apparatus (not shown) is employed for transmitting a high intensity, ultra high frequency electromagnetic wave which is reflected by a distant object, and where receiving apparatus (not shown) is connected to the transmission line 1. In such an instance, it will be appreciated that it is desired to protect the receiving apparatus during the transmitting operations, inasmuch as receiving apparatus is purposely designed in many instances to operate in response to relatively weak signals and consequently should be protected. In such a system, the resonant slot and associated structure serve to prevent the impression of the high intensity transmitted signal upon receiving apparatus when the transmitter is operating. This is effected by the protective action of the resonant aperture which breaks down by virtue of the high intensity signals of the transmitter, the waves not being transmitted beyond the wall Ii in the direction of line 1. Upon interruption of the transmitted signal, the electric discharge is immediately extinguished, thereby restoring the resonant condition of the slot i2 to permit the propagation through the guide of the relatively weak received or reflected signal.

While I have shown and described my invention as applied to a particular system and as embodying various devices diagrammatically shown, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a dielectric wave guide of the hollow-pipe type having a transverse cross sectional configuration to permit the propagation of electromagnetic waves above a predetermined frequency therethrough, a wall within said guide and lying in a plane substantially transverse to the direction of wave propagation there through. having an aperture tuned substantially to a frequency equal to or greater than said predetermined frequency, and an enclosure for said aperture including a. metallic member extending through said wall and surrounding said aperture and a pair of dielectric windows sealed to said member.

2. In combination, a dielectric wave guide of the hollow-pipe type having a transverse cross sectional configuration to permit the propagation therethrough of electromagnetic waves above a predetermined frequency, a wall within said guide and lying in a plane substantially transverse to the direction of wave propagation therethrough and having an aperture tuned substantially to a frequency equal to or greater than said predetermined frequency, and a reflectionless enclosure for said aperture including a metallic cylinder extending through said wall and surrounding said aperture and a pair of bulbous dielectric windows sealed to said cylinder.

3. In combination, a dielectric wave guide of the hollow-pipe type having a transverse cross sectional configuration to permit the propagation therethrough of electromagnetic waves above a predetermined frequency, a, wall within said guide and lying .in a plane substantially transverse to the direction of wave propagation therethrough and having an aperture tuned substantially to a frequency equal to or greater than said predetermined frequency, and an enclosure for said aperture including a metallic cylinder passing through said wall and extending an appreciable distance on each side thereof surrounding said aperture and a pair of bulbous vitreous windows sealed to said cylinder on opposite sides of said wall.

4. In combination, a dielectric wave guide of the hollow-pipe type, exciting means for establishing electromagnetic waves therein, a wall within said guide and lying in a plane substantially transverse to the direction of wave propagation therethrough and having therein an aperture tuned substantially to the frequency of said exciting means, and an enclosure for said aperture including an elongated member extending through said wall and surroundin said aperture, and a pair of vitreous windows sealed to said member and on opposite sides of said wall.

5. In combination, a dielectric wave guide of the hollow-pipe type, exciting means for establishing electromagnetic waves therein, a. wall within said guide and lying in a plane substantially transverse to the direction of wave propagation therethrough and having therein an aperture tuned substantially to the frequency of said exciting means, and an enclosure for said aperture having a reflection characteristic substantially equal and opposite in effect to that of said aperture and including a metallic member extending through said wall and surrounding said aperture and a pair of vitreous windows sealed to said member.

6. In combination, a dielectric .wave guide of the hollow-pipe type, exciting means for establishing electromagnetic waves therein, a wall Within said guide and lying in a plane substantially transverse to the direction of wave propagation therethrough and having therein an aperture tuned substantially to the frequency of said exciting means, and an enclosure for said aperture including a metallic cylinder extending through said wall and surrounding said aperture and a pair of vitreous windows spaced on opposite sides of said wall and sealed to said cylinder.

7. In combination, a dielectric wave guide of the hollow-pipe type, exciting means for establishing electromagnetic waves therein, a wall within said guide and lying in a plane substantially transverse to the direction of wave propagation therethrough and having therein an aperture tuned substantially to the frequency of said exciting means, and an enclosure for said aperture having a reflection characteristic substantially equal and opposite to that of said aperture and comprising a metallic cylinder extending through said wall and surrounding said aperture and a pair of bulbous vitreous windows on opposite sides of said wall and sealed to said cylinder. 8. In combination, a dielectric wave guide of the hollow-pipe type, exciting means for establishing electromagnetic waves therein, a wall within said guide and lying in a plane substantially transverse to the direction of wave propagation therethrough a d having therein an aperture tuned substantially to the frequency of said electromagnetic waves for effecting a concentration of the potential difference due to the electromagnetic waves to effect the establishment of an electric discharge thereacross when the intensity of the electromagnetic waves attains or exceeds a predetermined value, said wall being provided with an opening extending around said aperture, and an enclosure for said aperture including a metallic member passin through said opening and extending an appreciable distance on each side of said wall and a pair of vitreous windows sealed to said member.

9. In combination, a dielectric wave guide of the hollow-pipe type, exciting means for establishing electromagnetic waves therein, a wall within said guide and lying in a plane substantially transverse to the direction of wave propagation therethrough and having therein an aperture tuned substantially to the frequency of said exciting means and, provided with an appreciable dimension substantially perpendicular to the electric component of the electromagnetic field for effecting a concentration of the potential difference due to the waves within said guide, said wall being provided with a circular opening of a diameter greater than the principal dimension of said aperture, and an enclosure for said aperture comprising a metallic cylinder passing through said opening and having an appreciable distance on each side of said wall and. a pair of bulbous vitreous windows sealed to said cylinder.

10. In combination, a dielectric Wave guide of the hollow-pipe type, exciting means for establishing electromagnetic waves therein, a wall within said guide and lying in a plane substan- 10 tially transverse to the direction of wave propagation therethrough and having therein an aperture tuned substantially to the frequency of said exciting means and provided with an appreciable dimension substantially perpendicular to the electric component of the electromagnetic field for efiecting a concentration of the potentia1 difierence due to the waves within said guide, saidwall being provided with a circular opening of a diameter greater than the principal dimension of said aperture, and an enclosure for maintaining said aperture within a desired atmosphere at a predetermined pressure comprising a metallic member extending through said opening and a pair of bulbous vitreous windows sealed to said member.

KENNETH C. DE WALT. 

